1. Field of the Invention
The present invention relates to a dry grinding system which is suitable for use in production of, for example, abrasives or fillers; and to a dry grinding method employing the system.
Priority is claimed on Japanese Patent Application No. 2002-304390, filed Oct. 18, 2002, the content of which is incorporated herein by reference.
2. Description of Related Art
In general, ceramic powder such as alumina powder or silicon carbide powder, which is employed as, for example, abrasives or fillers, is produced through grinding of raw material powder having a large average particle size. Grinding processes are roughly classified into batch grinding processes and continuous grinding processes. Continuous grinding processes are roughly classified into an open-circuit grinding process and a closed-circuit grinding process. Among these grinding processes, a continuous grinding process; particularly, a closed-circuit grinding process, is widely employed by virtue of its excellent grinding efficiency. Grinding processes include a dry grinding process and a wet grinding process. When a dry product is to be produced by means of a grinding process, in many cases, a dry grinding process, which does not require a drying step, is employed.
An exemplary dry closed-circuit grinding system is described in “Chemical Engineering Handbook” published by Maruzen, Oct. 25, 1978, page 1265. The grinding system will next be schematically described with reference to FIG. 4.
As shown in FIG. 4, the conventional dry closed-circuit grinding system includes grinding means 120 for dry-grinding a material to be ground 110; classification means 130 for classifying a ground product 121 obtained through the grinding means 120, into fine powder 131 having a relatively small average particle size and coarse powder 132 having a relatively large average particle size; and returning means 140 for returning to the grinding means 120 the coarse powder 132 obtained through the classification means 130.
In this system, the fine powder 131 obtained through the classification means 130 is collected, and the coarse powder 132 is repeatedly subjected to grinding until the resultant powder attains a predetermined average particle size. The above-collected fine powder finds utility in a variety of applications without any further treatment, or, if desired, after being subjected to further classification.
However, the aforementioned conventional dry grinding system sometimes fail to attain an efficient production of a powder product having a target average particle size.
Alumina powder suitable for use as abrasives, etc. has an average particle size of, for example, 45 to 90 μm. In the case where an alumina powder product having such an average particle size is to be produced by subjecting, to further classification, alumina fine powder obtained through the classification means of the aforementioned conventional system, when a medium size crusher is employed as the grinding means, the fine powder contains large amounts of particles having a particle size greater than a target particle size; i.e., the amount of particles having a particle size falling within a target particle size range is reduced, and therefore the productivity is low.
Employment of a pulverizer as the grinding means should be a possible solution for increasing the amount of particles contained in the fine powder that have a particle size falling within a target particle size range. However, in this case, the fine powder contains large amounts of ultrafine particles, and thus classification efficiency of the fine powder is lowered, leading to low productivity.